Radial piston devices have outer piston shoes which do not enter into the cylinder. That is the common design of past and present. In my U.S. Pat. No. 3,223,046 the entering piston shoe was introduced, which elongated the piston stroke and thereby increased the power of the device of equal outer diameter and weight. It did so by partially entering the piston shoe into the respective cylinder through the form of an "H" of the piston shoe's outer portion.
My U.S. Pat. No. 3,277,834 thereafter introduced the "deep-diving" piston shoe. The deep diving of the piston shoe further extended the stroke of the piston of a device of a given diameter and weight and thereby drastically increased its power still further. This success was reached thereby, that the piston stroke actuator ring obtained an inner annullar groove for the reception of the outer portions of segments of the rotor. The "H-formed" deep diving piston could thereby fully enter into the respective cylinder beyond the outer diameter of the outer rotor portions. The increase in power of the devices of given diameter and weight increased so drastically thereby, that the power of a device of given dimension and weight multiplied.
Over the years of intensive research these devices with deep diving piston shoes were improved to deliver more and more power per unit size. Especially they were also developed as motors. Motors can succeed only with the very large piston strokes compared to the inner diameter of the actuator ring, which is called "da". The relation of stroke "s" to said "da", namely the relation S/da defines basically the power and quality of such a device.
With the ever increasing power and strokes the guide-length of the piston did however not increase enough. That led, as is now discovered, thereto, that the piston's outer end is pressed stronger against the wall of the cylinder, than the inner end of the piston, because of the location of the swing point or swing axis of the pivotion assembly of piston and piston shoe in the neighborhood of the outer end of the piston.